Observable classes provide a way in which data bound UI can be notified of changes.
ObservableList and ObservableMap also provide the ability to notify when
changes occur. ObservableField, ObservableParcelable, ObservableBoolean, ObservableByte,
ObservableShort, ObservableInt, ObservableLong, ObservableFloat, and ObservableDouble provide
a means by which properties may be notified without implementing Observable.